How to check a car's service history?

First thing would be to check and find the service book for the car. Usually this is found in the glove box. Some owners will bring this into their house to keep it for safe keeping. If there is no service book, you can check with the manufacturer, or ask the owner where they have had it serviced, as the garage may have a digital copy of the service history. Some more modern cars have service history either online, or stored on the vehicle's onboard computer, for example, BMW's.